Gildas (d. c.570), British ecclesiastic and monastic reformer, exercised considerable influence on the early Irish church. His ‘De excidio et conquestu Britanniae’, although not an historical work, remains a primary source for the history of post-Roman Britain. There, Gildas states (ch. 26) that his birth coincided with the year in which the Britons won a great victory over the Germanic (Anglo-Saxon) invaders at the battle of Mount Badon, which took place c.500. The last years of the generation of peace and calm that ensued coincided with the writing of ‘De excidio’, which was almost certainly written before 550, having taken possibly more than ten years to write and being completed in Gildas's forty-fourth year. The bulk of the work is a polemic in biblical style castigating the British rulers of the day for their tyranny and depravity. The text was widely circulated during the middle ages; among the authors who used it as a source was Bede, who drew on it for his ‘Historia ecclesiastica’.
A collection of seven fragments of letters by Gildas is preserved largely in the Irish collection of canons known as ‘Collectio Hibernensis’. The letters deal with questions of morality and discipline among monastic and non-monastic clergy and laity. They may have been written in response to a series of questions addressed to Gildas by ‘Vennianus’, most likely Finnian (qv) of Movilla. According to Columbanus (qv), Gildas ‘replied in a most elegant style’, a fact attested by the latinity of the extant fragments. While he may not have had episcopal status, Gildas was certainly a priest; Finnian would not have consulted him had he not been a monastic leader of considerable reputation, and therefore a man of mature years. The fragments are a remarkable survival from the early middle ages and provide eloquent testimony to the influence of Gildas on the early Irish church.
Gildas's authorship of a penitential commonly attributed to him has been disputed, some notable differences between the letters and the penitential leading some to question their common authorship. Allowing for the very different contexts and purposes of the letters and the penitential, however, it is indeed possible that Gildas wrote both. Apart from ‘De excidio’, the other works of Gildas owe their preservation to the sustained interest of the early Irish church and the high esteem in which the monastic movement, of which he was a notable reformer, was held in Britain and Ireland.